Telephone system



Dec. 23, 1930. G. `H. PETERSON ET AL 1,785,818'

TELEPHONE SYSTEM Filed Jan. 9. 1929 w s B w @l1/Erikson wf/ww WA /T/vaafs Patented Dec. 23, 1930 ju-Nirelisierras PATENT OFFICE GEORGE H. rE'rEEsomoF -BLOOMFIELn NEW JERSEY, ANDWILLIAM A. RHODES, OE

Penn A CORPORATION OE NEW YORK i 'NEw jroEK,`N.' Y.,'As`srGN'oEs VToV AMERICAN TELEPHONE AND TELEGRArH OOM- p k'r`Erlii-1=.HONE SYSTEM Application filed January 9, 1929. Serial No331,209.

' v"This invention relates yto telephonesystems providing a hunting switch, three relays, and

and ,more .particularly to "automatic selec-` a key, similar to the equipment shown to tion ofidle order wiresfinteroiicetrunk circuits and the like. l .l 7 I f automatic switches are employed for completing a talking connection.: between :two A.

` operators, the switch contacts are only called e upon to pass the* relatively small Speech/eur rents. It has Abeenzknown for some time that a high resistance iilm or oXidi'zation is, apt to' form upon, rubbing/contacts composedV `of base metals 'which i'vsfoffsucliih'igh resistance j astol seriouslyinterferei with speech trans, mission and vit hasheretofore been proposed 'to break down: such filmor-oxidizationfby' means of an auxiliary `current of sucha high value as'to'dissipate such resistance. g

In previous Contact cleaning systems, as

shown, e for example, Ain the German", Patent Q 277,368, issued AAugust 10, 1914, the .so-called contact cleaning current is appliedto the contacts 1nv series *and under certain-circuit conditions `objectionable clicks wouldY be heard by' thetoperators, at each end O f/they circuit, if the talking path were interrupted during the time the cleaning current circuit wasclosed. ,k' y I An object of the present invention there fore is to prevent objectionableclicks in the operators' receivers: associated with 'circuits employingsuch Contact cleaning means as before described. A In accordance with thefforegoing object, a

' feature of the invention resides 4incompleting a rcircuit, for' the sourcey of'y cleaning current` potentiall Over"the g switch f contacts i-n -par-jV .,allel instea'dofinseries,l Y "f f rThe invention will be understood fromthe following 'descriptionfwhen read in'connection yfwithf thei accompanying? 'drawing'T in which-an A'opera'torsposition A may be connected -witlr-an *idle Bqoperators telephone AB j over;l a callcircuitf-hunting switch; Y

- ff for a direct current flow over the tip and ring group extendfrom-the-switch to'jother B Operators; not shownf theleft of the broken line XY, for each adn p y e fditional group of call circuits to which the Incertain circuit arrangements in whichV A operator'is tohave access.

The operation of the system, according to the present invention, will now be described. lVhen the A operator desiresfto connect with a B operator reached over the group of call circuits represented by CC, CO2, etc., she :will depress the corresponding call circuit key K thereby Operating the slow release relay ST which prepares a circuit for the subsequent operation of relay CI and, in addition, closes a circuit for marginal relay TST. The TST relay will Operate if the brushes of switch H rest on busy terminals, which' will be indicated by ground on conductor l. The absence of lground on conductor 1, as shown on the'drawing, will permit relay CI to opera-te in a circuit which can be traced fromground at the left-hand contacts 2 and 3 of rela-y ST, winding of relay CI, contacts ductor 1 through the switch brush HS thereby rendering the B position reached over call circuitCC busy to all other hunting switches. It will be noted that the 'Operation of the T relay also connects ground, at its contacts 2, to the midpoint of the right-hand winding of trai'isformer 7 and'it will be 'later exi-V plained how this ground completesa circuit.

conductor of the call circuit CC and through the switch contacts, whereby satisfactory speech transmission is insured. y

If it is assumed, as previously mentioned and shown on the drawing, that thebrushes of switch H are standing on the first contact of the bank, to which is connected the call circuit CC, ground is connected to sleeve conductor 1 over contacts 2 and 3 of relay ST,

contacts 6 of relay Cl, and brush HS VYof switch H, to causethe operationyof relay S at the distant B position. l

It will be further assumed that the distant 1B operator is at her position ai'id'has'coiiner-,ted her telephone set ,in the usualmanoei" to the position circuit thereby causing operationof relay T, the circuit for which can be traced from the transmitter, battery 28 through the winding of relay T, the B operators transmitter, and right-hand ywinding of induction coil 8 to ground. A l

When relay S operated, it connected ground atits contacts 9 vto start the` operation of tone relays TB, TC, TD and TE whose function is to transmit two short impulses of tone current back over the call circuit to the A operator as a signal that the B operator is atl her position and ready to receive the call. The operation of these relays Ground supplied at contacts 9vof relay S passes over contacts 10` of relay TA and through the winding of relay TB to battery. Relay TB operates in this circuit and connects ground at its contacts 12, over contacts 13 of relay TCto complete' the circuit for interrupted battery through the primary winding of transformer 14 thereby causing audi'- ble'frequency alternating current tobe invduced in the secondary winding of the transformer which isy connected to the call circuit over contacts 15 and 16 of relay TA. As soon,

however, as relay TB operates, it also extends its operating ground to the Winding of relay TC rwhich in turn operates and opens at its i contacts 13, the circuit through the primary p winding of transformer 14 thereby discontinuing the induced tone current. Relay TC in operating further, extends at its left-hand contacts, operating groundy for relay TDV which again closes the interrupted battery circuit through the primary Winding of transformer 14 thereby' again transmitting audible tone current tothe call circuit. TD in operating, also extends its operating ground over its left-hand contacts to operate relay l TE, which in turn connects ground to opersignal to pass the call as a talking circuit now existsbetween theA and B operators over the transformer 7, contacts of relay/ACI, contacts of switch H, contacts of relay T,

transformer 17, contacts of relay TA and inis as follows :7

set.

duction coil 8 of the B operatorstelephone 'i In case the B operator is absent'V-froin her position, relay T will not be operated and although the tonerelays TB, TC, etc.- will function in the .previously described Vmanner,

, should the A operator connect to the call circuit CC the callv assinotones Generated in transformer 14 will not be transmitted to;Y Y

the cally circuit dueto the fact fth'at thelr'elay contacts 18 and 19 of relay T are opengV Seizure of the call circuit CC will, however, cause the operation of night alarm relay NA which completes an obvious circuit for lightingthe pilot lamp 20.

vAs-.previouslyrv referred to, it willbe noted thatwhen relayS at the B position operates, it closes af circuit from battery, through n contacts 21 of relay S, and resistance 22 to themid-'point of the left-'hand winding of transformer 17', kthereby causingV a current flow of the order of 25 mils over the tip and ring conductors of call circuit CC', in paral-` lel, to ground at the midpoint lof the-righthand winding` of transformer 7 latthe A operators position.l

The reason for this arrangement is that itY has been'found that a high resistance film of oxidation is'apt to form on the'. base metal contact surfaces of selector switches which is apt tol causelossesin the transmission of speech vin cases where dry circuits are employed, .that is, circuits overl'which battery current does not actually flow. To overcome this condition, it has been found advisable to Wet the switch contacts by means of a current of the order of 25 mils during the conversation period and in the present circuit arrangement, this current isvpassed over the tip and ring contacts of the switch in parallel, instead of in series, thereby'preventingl clicks in both the B and A operators re-V ceivers when the connection is broken down,

`which would be caused by` inductive discharges through the-windingsof transformers 7 and 17 in case batteryy was serially connected through the line windings of trans formers 7 and 17.

What is claimed is:

1. In a selecting circuit a two-conductor line extending between two stations,switch ing apparatus contacts' ineachV conductor of said'line, said contacts having a high resist-""1` ance film over their contact surfaces, a source of electrical current of sufficient magnitude to eliminate said contact resistance and means responsive to talringsaid line for use at one station for completing a circuit for said current source yover said line conductors and contacts in parallel. f f

2. `In a selecting circuit a two-conductorline extending between two stations, switching apparatus contacts in each conductor of said line, said contacts having a high resistance oxidation film A'over their contact sur'- 

